Heterosexual transmission of HIV-1 remains the dominant mechanism by which the epidemic is sustained worldwide. In a majority of transmissions, infection is established by a single genetic variant (the transmitted founder, TF) from the quasispecies of the transmitting partner (TP), with evidence for reduced Env glycosylation relative to their chronic counterparts. This concept of selection of virus with traits that favor transmission has been supported by our recent studies of viruses in the genital tract (GT), but it remains to be determined what functional properties differentiate TF viruses from their transmitting partner counterparts We hypothesize that differences in TF virus proteins allow preferential infection of key cell types within the genital mucosa and through interactions with antigen presenting cells signal the influx of activated CD4 T cells that can home to gut tissues. In this context, we will generate and characterize full-length single HIV-1 TF virus genomes and genomes amplified from TP bloodand GT in 20 Rwandan and Zambian transmission pairs; determine their abilities to productively infect vaginal and cervical tissue explants, determine whether different cells are infected by donor and recipient viruses, investigate their ability to interact with alpha-4, beta-7 molecules on mucosal T-cells and to induce the release of cytokines in antigen presenting cells capable of signaling trafficking of relevant activated T-cell populations. Newly infected partners in couples infected by different strains of HIV-1, are at high risk for superinfection and subsequent virus recombination. Based on preliminary data, we hypothesize that susceptibility to superinfection is related to low levels of neutralizing antibodies immediately prior to superinfection and that studies of superinfection will inform on the nature of protection required for an effective vaccine. We will therefore determine the frequency, kinetics and the virologic/immunologic ramifications of HIV superinfection in this cohort, as well as determine whether specific immunologic defects, such as low levels of protective antibody contribute to susceptibility to superinfection. The results of these studies will yield novel information that is critical to the design and testing of globally effective vaccine candidates.